Various analytical procedures and devices are commonly employed in assays to determine the presence and/or concentration of substances of interest or clinical significance which may be present in biological fluids or other materials. Such substances are commonly termed "analytes" and can include antibodies, antigens, drugs, hormones, etc. The detection of particular analytes in biological fluids such as serum, plasma, urine, spinal fluid and the like has in recent years become of critical importance in both research and clinical settings. The detection of analytes of interest can often be related to various disease states and consequently is extremely useful in the diagnosis of disease and in monitoring the effectiveness of therapy. When the analytes are antigens or antibodies, assays typically depend upon the immunological reactivity which characterizes these substances. Generally, such assays are collectively termed immunoassays.
Immunoassay techniques take advantage of the mechanisms of the immune systems of higher organisms, wherein antibodies are produced in response to the presence of antigens which are pathogenic or foreign to the organisms. At least one antibody is produced in response to and is capable of reacting with a particular antigen, thereby creating a highly specific reaction mechanism which can be used in vitro to determine the presence or concentration of that antigen in a biological sample. When immunoassay techniques are used to determine the concentration of an antigen in a biological sample, standards containing different known amounts of the antigen can be prepared and subjected to the same test procedure as the biological sample; the concentration of the antigen in the sample can then be determined by comparing the magnitude of the response (for example, the rate of fluorescence in the case of a known assay for prolactin) obtained from the sample with the magnitude of the responses obtained from the standards.
Immunoassays for the determination of prolactin in blood, serum and the like are known, a synthetic peptide which can be used in such immunoassays being disclosed in U.S. Pat. No. 4,585,740, granted Apr. 29, 1986, and the use of a radio-iodinated hormone stabilized with a non-ionic detergent in such immunoassays being disclosed in U.S. Pat. No. 4,357,310 (see, also, journal articles by Wide, L. et al.: Immunochemistry 10 4:381-386, 1967 and Acta Endocrinol., suppl. 174:1-58, 1973).
The aqueous calibrator composition of the instant invention can be used to prepare standards containing different known amounts of prolactin as described above. By determining the rate of fluorescence of the standards, it is possible to prepare a curve of prolactin content as a function of the rate of fluorescence and to determine the prolactin content of a biological sample by comparing its rate of fluorescence with the curve.
It is known that prolactin solutions are unstable in the sense that prolactin assayed after prolonged storage under most storage conditions will be a fraction (often one-half or less) of that before storage. For example, Nyberg et al. (Upsala J Med Sci 90:265-277, 1985) report, at p. 274:
"Prolonged storage (2-3 years) at a protein concentration in the range 0.1-2 mg/ml in 0.02 M Tris-HCl buffer (pH 9) at -80.degree. C. neither affected the immunopotency nor altered the electrophoretic pattern of prolactin." PA0 "(d) in 0.02 M Tris-HCl buffer (pH 9.0) at -20.degree. C. PA0 "(e) in 0.01 M ammonium bicarbonate (pH 7.8) at -20.degree. C. PA0 "(f) in 0.02 M Tris-HCl buffer (pH 9.0) containing 0.5 M KCl" (storage temperature not specified).
On the same page, the authors present a plot showing that the preserved prolactin activity was less than fifty percent twenty weeks after preparation when stored:
Because of the instability of prolactin under most storage conditions, it is common practice to furnish lyophilized prolactin, shipped and stored under ambient conditions, and used just before an immunoassay is conducted to prepare calibrators.